Thanks, that makes sense.  And it at least less ugly than the __new__ hack I was thinking of as a workaround.  though similar in spirit.

presumably self.append should get the same treatment for completeness given the PEP 307 text.

I didn't notice a place in the Python pickle docs that mentioned this specific thing about listitems and the need for append/extend.  I didn't do a thorough read though, maybe I overlooked something?  If not, it'd be worth figuring out how to get this uninitialized class calling of append/extend/__setitem__ for list and dict objects detailed in the main docs rather than off in the PEP.

It's semi-mentioned https://docs.python.org/3/library/pickle.html#object.__reduce__ here which is what the PEP added I suppose, but given this code has no custom __reduce__, we need to explicitly mention that list and dict subclasses supporting pickling/copying may need to be prepared to handle this situation.  With a versionchanged:: 3.7 noting that it now always applies to list subclasses without their own __reduce__.